1. Technical Field
The present invention relates to a laminated wave plate in which two wave plates formed of an inorganic crystalline material such as quartz crystal having birefringence properties are arranged to overlap with each other, and an optical pickup device, a polarization converter, and a projection display apparatus which employ the laminated wave plate.
2. Related Art
A half-wave plate emitting as an output beam a linearly-polarized beam obtained by rotating a polarization plane of a linearly-polarized beam as an incident beam by a predetermined angle, for example, 90°, has been employed in optical devices such as an optical pickup device used for recording on and reproduction from an optical disk device, a polarization converter, and a projection display apparatus such as a liquid crystal projector. A wave plate formed of a single plate serves as a half-wave plate at a predetermined wavelength, since the phase difference thereof is determined depending on the thickness thereof. A laminated wave plate in which two or more single wave plates are bonded so that optical axes thereof intersect has been developed which serves as a half-wave plate in a broader wavelength range (for example, see JP-A-11-149015).
The optical pickup device used for recording on and reproduction from an optical disk device employs a violet-blue laser beam with a very short wavelength and high power so as to increase the recording density and the capacity thereof. In the liquid crystal projector, durability and long-term reliability are required for a half-wave plate as an optical component of an optical engine with elongation in the lifetime of the optical engine.
However, the half-wave plate described in JP-A-11-149015 has a structure in which plural sheets of stretched films obtained by stretching a polymer film formed of polycarbonate or the like to give a phase difference of a half wavelength thereto are laminated. Accordingly, the polymer film of the half-wave plate may absorb the violet-blue laser beam and may emit heat and the material itself may deteriorate, thereby damaging the function of the wave plate. On the contrary, since an inorganic crystalline material such as quartz crystal or calcite has very high light resistance, a wave plate formed of quartz crystal or the like can be advantageously used in an optical system employing the violet-blue laser beam.
Optical disk recording and reproducing apparatuses require a function of enabling the recording on and reproduction from recording mediums such as a Blu-ray disk, a DVD, and CD based on different standards. In general, the Blu-ray disk is used at a wavelength band of 405 nm, the DVD is used at a wavelength band of 660 nm, and the CD is used at a wavelength band of 785 nm. Accordingly, it is preferable that the half-wave plate of the optical pickup device operates at all or two wavelength bands of the above-mentioned wavelength bands. On the other hand, the half-wave plate used in the polarization converter of the liquid crystal projector needs to maintain a phase difference of 180° in a broad wavelength range of 400 nm to 700 nm.
In general, since a half-wave plate has wavelength dependence that phase difference varies with a variation in wavelength, the phase difference increases or decreases in wavelength bands in the vicinity of a target wavelength. Therefore, a laminated wave plate has been suggested (for example, JP-A-2004-170853) the whole of which serves as a half-wave plate in the broad wavelength range of 400 to 700 nm and is formed by bonding a first wave plate with an optical axis bearing angle θ1 and a second wave plate with an optical axis bearing angle θ2 so that the optical axes thereof intersect each other and satisfy the relations of θ2=θ1+45° and 0<θ1<45°.
When a beam emitted from a light source is incident on the half-wave plate, there is a problem with the incident angle dependence that the phase difference varies in regions other than the vicinity of the center of the wave plate. Accordingly, a polarization conversion efficiency of the half-wave plate, that is, a ratio at which the incident linearly-polarized beam of P polarization (or S polarization) is converted into a linearly-polarized beam of S polarization (or P polarization) and the resultant beam is output, is lowered, thereby causing a loss of light intensity. Therefore, a high-order-mode laminated wave plate is known in which first and second wave plates with a phase difference of 180°+360°×n (where n is a positive integer) are bonded so that the optical axes thereof intersect each other and θ2=θ1+θ/2 is satisfied, where in-plane bearing angles of the first and second wave plates are represented by θ1 and θ2 and an angle formed by the polarization direction of the linearly-polarized beam incident on the laminated wave plate and the polarization direction of the linearly-polarized beam output therefrom is represented by θ (For example, JP-A-2007-304572). By setting n=5, θ1=22.5°, and θ2=67.5° in the laminated wave plate, the wavelength conversion efficiency with the variation in wavelength can be set to almost 1 in three wavelength bands of 405 nm, 660 nm, and 785 nm, thereby suppressing the loss in light intensity.
Similarly, to improve the polarization conversion efficiency, a laminated wave plate is suggested which serves as a half-wave plate in which a first wave plate with a phase difference Γa=180° and a second wave plate with a phase difference Γb=180° are bonded, the optical axis bearing angles θa and θb of the first and second wave plates satisfy θb=θa+α, 0<θa<45°, and 40°<α<50°, and a difference ΔΓa of the phase difference Γa from a designed target value and a difference ΔΓb) of the phase difference Γb from a designed target value satisfy a predetermined relational expression (for example, see JP-A-2008-268901). In this laminated wave plate, by canceling the difference ΔΓa of the phase difference Γa from the designed target value with the difference ΔΓb of the phase difference Γb from the designed target value on the basis of the predetermined relational expression, it is possible to obtain a high polarization conversion efficiency.
FIGS. 15A and 15B are diagrams illustrating a typical example of the above-mentioned laminated half-wave plate according to the related art. The laminated half-wave plate 1 includes first and second wave plates 2 and 3 which are formed of an optical uniaxial crystalline material such as a quartz crystal substrate and which are arranged sequentially from the light incidence side Li to the light output side Lo. The first and second wave plates 2 and 3 are single-mode half-wave plates with phase differences of Γ1=180° and Γ2=180°, respectively, and are bonded so that crystal optical axes 4 and 5 thereof intersect each other at a predetermined angle. Here, the optical axis bearing angle θ2 of the first wave plate 2 is an angle formed by the crystal optical axis 4 and the polarization plane of a linearly-polarized beam 6 incident on the laminated half-wave plate 1 and the optical axis bearing angle θ2 of the second wave plate 3 is an angle formed by the crystal optical axis 5 and the polarization plane of the linearly-polarized beam.
In the laminated half-wave plate 1 shown in FIGS. 15A and 15B, the optical axis bearing angles of the first and second wave plates 2 and 3 are set to θ2=22.5° and θ2=67.5°, respectively, and the angle formed by the polarization direction of an incident linearly-polarized beam 6 and the polarization direction of an output linearly-polarized beam 7 is set to 90°. The polarization state in this case is described now using a Poincare sphere shown in FIGS. 16A to 16C. FIG. 16A is a diagram illustrating a trajectory transition in the Poincare sphere of the linearly-polarized beam incident on the laminated half-wave plate 1. A position in the equatorial line at which the linearly-polarized beam 4 is incident is set to an intersection point P0 with an axis S1. FIG. 16B is a view illustrating the locus of the polarization state of a beam incident on the laminated half-wave plate 1 as viewed from an axis S2 in the Poincare sphere shown in FIG. 16A, that is, a projected diagram onto the plane S1S3. FIG. 16C is a view illustrating the locus of the polarization state of a beam incident on the laminated half-wave plate 1 as viewed from an axis S3 in the Poincare sphere shown in FIG. 16A, that is, a projected diagram onto the plane S1S2.
The reference point of the incident beam is set to a point P0=(1, 0, 0), the rotation axis R1 of the first wave plate 2 is set to a position which is rotated from the axis S1 by 2θ1, and the rotation axis R2 of the second wave plate 3 is set to a position which is rotated from the axis S1 by 2θ2. When the reference point P0 is rotated about the rotation axis R1 to the right side by the phase difference Γ1, the point P1=(0, 1, 0) in the equatorial line of the Poincare sphere is the position of the output beam of the first wave plate 2. When the point P1 is rotated about the rotation axis R2 to the right side by the phase difference Γ2, the point P2=(−1, 0, 0) in the equatorial line of the Poincare sphere is the position of the output beam of the second wave plate 3, that is, the position of the output beam of the laminated half-wave plate 1. As long as the wavelength of the incident beam Lo does not depart from the target value, the position of the output beam is located in the equatorial line of the Poincare sphere.
However, an optical pickup device mounted on a Blu-ray optical disk recording and reproducing apparatus employs a short-wavelength (405 nm) violet-blue laser. When the violet-blue laser expands due to the high temperature when being used, a problem is caused in that the wavelength of an oscillated laser drifts (varies). Accordingly, in the optical pickup device, the half-wave plate causes a problem that the conversion efficiency of the linearly-polarized beam is deteriorated due to the wavelength drift of the incident laser beam.
In the optical disk recording and reproducing apparatus enabling the recording and reproduction of both a Blu-ray disc and a DVD, the wavelength drift of the laser beam may occur in one of the used wavelength bands of 405 nm and 660 nm. Accordingly, in the half-wave plate used in the optical disk recording and reproducing apparatus, it is necessary to suppress the deterioration in conversion efficiency with the variation in wavelength in both bands.
JP-A-2004-170853 discloses a method for preventing or reducing the influence of the variation in wavelength. In this method, when the differences of the phase differences of the first and second wave plates due to the variation in wavelength are ΔΓ1 and ΔΓ2, the differences of the phase differences can be canceled by setting ΔΓ1=ΔΓ2. Accordingly, the position P2 of the output beam in the Poincare sphere is always located in the equatorial line.
This will be described using the Poincare sphere shown in FIGS. 16A to 16C. The position of the output beam of the first wave plate 2 is the point P1′ rotated about the rotation axis R1 from the point P1 to the right side by the difference ΔΓ1. The position of the output beam of the second wave plate 3 is the point P2′ in the equatorial line of the Poincare sphere which is rotated about the rotation axis R2 from the point P1′ to the right side by the difference Γ2+ΔΓ2. The point P2′ is the position of the output beam from the laminated half-wave plate 1. As can be seen from the drawings, since the point P2′ is deviated from the point P2 in the equatorial line, the rotation of the polarization plane of the output beam is deviated from 90°. As described in JP-A-2004-170853, since the rotational deviation of the polarization plane of the output beam can be less influenced as ΔΓ1 and ΔΓ2 become less, it is preferable that the first and second wave plates 2 and 3 are formed of single-mode wave plates, respectively, thereby reducing the wavelength dependence as much as possible.
JP-A-2008-268901 discloses a problem that the position of the output beam of the first wave plate in the Poincare sphere is deviated when the thickness processing accuracy of the first wave plate is deviated from the designed value. To solve this problem, JP-A-2008-268901 discloses a method of processing the thickness of the second wave plate so as to cancel the deviation of the position of the output beam of the first wave plate. However, in the laminated half-wave plate disclosed in JP-A-2007-304572, since the first and second wave plates are the high-order-mode wave plates, there is a problem in that the wavelength bandwidth in which the conversion efficiency is close to 1 is reduced when the order n is excessively increased, thereby making it difficult to use it as the laminated half-wave plate.
Here, the conversion efficiency is an estimated value used to accurately determine the polarization state of the output beam of the laminated half-wave plate including two wave plates bonded to each other, as described in JP-A-2007-304572, and is obtained by calculating the light intensity of the output beam with respect to the incident beam by a predetermined calculation technique. This method is simply described below.
In the laminated half-wave plate 1, when the Muller matrix of the first wave plate 2 is represented by R1, the Muller matrix of the second wave plate 3 is represented by R2, the polarization state of the input beam is represented by vector I, and the polarization state of the output beam is represented by vector E, the polarization state of the beam having passed through the laminated half-wave plate 1 can be expressed by the following expression.E=R2·R1·I  Expression 1Here, R1 and R2 are expressed by the following expressions.
                                              ⁢                  Expression          ⁢                                          ⁢          2                                                                                          R            1                    =                      [                                                  ⁢                                                            1                                                  0                                                  0                                                  0                                                                              0                                                                      1                    -                                                                  (                                                  1                          -                                                      cos                            ⁢                                                                                                                  ⁢                                                          Γ                              1                                                                                                      )                                            ⁢                                              sin                        2                                            ⁢                      2                      ⁢                                                                                          ⁢                                              θ                        1                                                                                                                                                        (                                              1                        -                                                  cos                          ⁢                                                                                                          ⁢                                                      Γ                            1                                                                                              )                                        ⁢                    sin                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                                          θ                      1                                        ⁢                    cos                    ⁢                                                                                  ⁢                    2                    ⁢                                          θ                      1                                                                                                                                  -                      sin                                        ⁢                                                                                  ⁢                                          Γ                      1                                        ⁢                    sin                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                                          θ                      1                                                                                                                    0                                                                                            (                                              1                        -                                                  cos                          ⁢                                                                                                          ⁢                                                      Γ                            1                                                                                              )                                        ⁢                    sin                    ⁢                                                                                  ⁢                    2                    ⁢                                          θ                      1                                        ⁢                    cos                    ⁢                                                                                  ⁢                    2                    ⁢                                          θ                      1                                                                                                                                  1                      -                                                                        (                                                      1                            -                                                          cos                              ⁢                                                                                                                          ⁢                                                              Γ                                1                                                                                                              )                                                ⁢                                                  sin                          2                                                ⁢                        2                        ⁢                                                  θ                          1                                                                                      )                                                                                        sin                    ⁢                                                                                  ⁢                                          Γ                      1                                        ⁢                    cos                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                                          θ                      1                                                                                                                    0                                                                      sin                    ⁢                                                                                  ⁢                                          Γ                      1                                        ⁢                    sin                    ⁢                                                                                  ⁢                    2                    ⁢                                          θ                      1                                                                                                                                  -                      sin                                        ⁢                                                                                  ⁢                                          Γ                      1                                        ⁢                    cos                    ⁢                                                                                  ⁢                    2                    ⁢                                          θ                      1                                                                                                            cos                    ⁢                                                                                  ⁢                                          Γ                      1                                                                                            ]                          ⁢                                  ⁢                                  ⁢                  Expression          ⁢                                          ⁢          3                                    (        2        )                                          R          2                =                  [                                                    1                                            0                                            0                                            0                                                                    0                                                              1                  -                                                            (                                              1                        -                                                  cos                          ⁢                                                                                                          ⁢                                                      Γ                            2                                                                                              )                                        ⁢                                          sin                      2                                        ⁢                    2                    ⁢                                                                                  ⁢                                          θ                      2                                                                                                                                        (                                          1                      -                                              cos                        ⁢                                                                                                  ⁢                                                  Γ                          2                                                                                      )                                    ⁢                  sin                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      θ                    2                                    ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                      θ                    2                                                                                                                    -                    sin                                    ⁢                                                                          ⁢                                      Γ                    2                                    ⁢                  sin                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      θ                    2                                                                                                      0                                                                                  (                                          1                      -                                              cos                        ⁢                                                                                                  ⁢                                                  Γ                          2                                                                                      )                                    ⁢                  sin                  ⁢                                                                          ⁢                  2                  ⁢                                      θ                    2                                    ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                      θ                    2                                                                                                                    1                    -                                                                  (                                                  1                          -                                                      cos                            ⁢                                                                                                                  ⁢                                                          Γ                              2                                                                                                      )                                            ⁢                                              cos                        2                                            ⁢                      2                      ⁢                                              θ                        2                                                                              )                                                                              sin                  ⁢                                                                          ⁢                                      Γ                    2                                    ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                                      θ                    2                                                                                                      0                                                              sin                  ⁢                                                                          ⁢                                      Γ                    2                                    ⁢                  sin                  ⁢                                                                          ⁢                  2                  ⁢                                      θ                    2                                                                                                                    -                    sin                                    ⁢                                                                          ⁢                                      Γ                    2                                    ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                      θ                    2                                                                                                cos                  ⁢                                                                          ⁢                                      Γ                    2                                                                                ]                                    (        3        )            
When the order n of the high-order mode of the first and second wave plates 2 and 3, the phase differences Γ1 and Γ2, and the optical axis bearing angles θ1 and θ2 are set, the Muller matrixes R1 and R2 are calculated by using Expressions 2 and 3, and the polarization state I of the incident beam is set, the polarization state E of the output beam is calculated by using Expression 1. The polarization state E of the output beam is called a Stokes vector and is expressed by the following expression.
                    Expression        ⁢                                  ⁢        4                                                            E        =                  [                                                                      S                  01                                                                                                      S                  11                                                                                                      S                  21                                                                                                      S                  31                                                              ]                                    (        4        )            
Here, the E matrix elements S01, S11, S21, and S31 are called Stokes parameters and indicate the polarization state. When the transmission axis of a matrix P of a polarizer is set to a predetermined angle and the product of the matrix E indicating the polarization state E of the output beam and the matrix P of the polarizer is T, T is expressed by the following expression.Expression 5T=P·E  (5)
The matrix T indicates the conversion efficiency and can be expressed by the following expression using the Stokes parameters of the elements.
                    Expression        ⁢                                  ⁢        6                                                            T        =                  [                                                                      S                  02                                                                                                      S                  12                                                                                                      S                  22                                                                                                      S                  32                                                              ]                                    (        6        )            
Here, when the Stokes parameter S02 of the vector T represents the light intensity and the incident light intensity is set to 1, the Stokes parameter S02 is the conversion efficiency. Accordingly, the conversion efficiency T of the laminated half-wave plate 1 can be simulated while variously changing the order n of the high-order mode of the first and second wave plates 2 and 3, the phase differences Γ1 and Γ2 at a predetermined wavelength (for example, at a wavelength of 405 nm), and the optical axis bearing angles θ1 and θ2.
FIG. 17 shows the simulation result of the variation in conversion efficiency T with respect to the wavelength of the incident beam using the calculation method when the designed wavelength λ0, which is used in the laminated half-wave plate 1 shown in FIGS. 15A and 15B, is changed to 400, 500, 600, 700, and 800 nm. In the drawing, it can be seen in any designed wavelength that the conversion efficiency has a high value of almost 1 in the vicinities of the designed wavelength and decreases as the wavelength gets farther from the designed wavelengths. When the laminated half-wave plate is used in the optical pickup device, the conversion efficiency of the incident linearly-polarized beam may decrease due to the wavelength drift of the laser beam.
FIG. 18 shows the simulation result of the variation in conversion efficiency T with respect to the designed wavelengths of the wave plates using the calculation method when the used wavelength band of the laminated half-wave plate 1 shown in FIGS. 15A and 15B is changed to 405±30, 660±30, and 785±30 nm. In the drawing, it can be seen in any wavelength band that the conversion efficiency has a high value of almost 1 in the vicinities of the target wavelength bands and decreases as the designed wavelength gets farther from the target wavelength bands. This states that it is difficult to make the conversion efficiency be 1 at plural discrete wavelength bands in the laminated half-wave plate according to the related art.